The Synchronous Optical Network (SONET) is a set of standards that define a hierarchical set of transmission rates and transmission formats for carrying high-speed, time-domain-multiplexed (TDM) digital signals. SONET lines commonly serve as trunks for carrying traffic between circuits of the plesiochronous digital hierarchy (PDH) used in circuit-switched communication networks. SONET standards of relevance to the present patent application are described, for example, in Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria (Telcordia Technologies, Piscataway, N.J., publication GR-253-CORE, September, 2000), which is incorporated herein by reference. While the SONET standards have been adopted in North America, a parallel set of standards, known as Synchronous Digital Hierarchy (SDH), has been promulgated by the International Telecommunications Union (ITU), and is widely used in Europe. From the point of view of the present invention, however, these alternative standards are functionally interchangeable.
The lowest-rate link in the SONET hierarchy is the OC-1 level, which is capable of carrying 8000 STS-1 frames per second, at a line rate of 51.840 Mbps. An STS-1 frame contains 810 bytes of data, which are conventionally organized as a block of nine rows by 90 columns. The first three columns hold transport overhead (TOH), while the remaining 87 columns carry the information payload, referred to as the synchronous payload envelope (SPE). The SPE contains one column of payload overhead (POH) information, followed by 86 columns of user data. The POH can begin at any byte position within the SPE capacity of the payload portion of the STS-1 frame. As a result, the SPE typically overlaps from one frame to the next. The TOH of each frame contains three pointer bytes (H1, H2, H3), which are used to indicate where in each frame the POH begins and to compensate for timing variations between the user input lines and the SONET line on which the STS-1 frames are transmitted.
STS-1 frames can efficiently transport the DS-3 level of the PDH, which operates at 44.736 Mbps. The STS-1 frames themselves are not too much larger than DS-3 frames. When PDH signals at rates below DS-3 are to be carried over SONET, the SPE of the STS-1 frame is divided into sections, known as virtual tributaries (VTs), each carrying its own sub-rate payload. The component low-rate signals are mapped to respective VTs, so that each STS-1 frame can aggregate sub-rate payloads from multiple low-rate links. Multiple STS-1 frames can be multiplexed (together with STS-Mc frames) into STS-N frames, for transmission on OC-N links at rates that are multiples of the basic 51.840 Mbps STS-1 rate.
For the purpose of VT mapping, each STS-1 frame is divided into seven virtual tributary groups (VTGs), each occupying 12 columns of the SPE. Within each VTG, four VT sizes are possible:                VT1.5—occupies three columns, each with sufficient bandwidth to transport a DS-1 signal at 1.544 Mbps (i.e., the signal carried on a T-1 line). One VTG can contain four VT1.5 sections.        VT2—four columns, bandwidth sufficient for an E-1 line.        VT3—six columns, bandwidth sufficient for DS-1C.        VT6—twelve columns, bandwidth sufficient for DS-2.Mapping of the VTs to the columns of the SPE is specified in detail in the above-mentioned Telcordia publication GR-253-CORE, section 3.2.4. It is not necessary that all of the VTs in a STS-1 frame be used to carry lower-rate signals. Unequipped VT sections, i.e. sections that have no service to carry, in the SPE are simply filled with default data. These unequipped sections are assigned a special indication in the VT POH byte V5, bits 5 to 7, known as the “signal label.” In SDH systems, STM-1 frames are similarly divided into sub-rate payload sections of different sizes, referred to as TU-11, TU-12 and TU-2.        
Circuit emulation services (CES) is a developing technology for transporting SONET and legacy PDH signals over packet networks, such as Internet Protocol (IP) networks. CES allows the network operator to maintain existing TDM service interfaces in a manner transparent to network users, even when the data traffic travels through a core packet network. For example, the CES operator could continue to offer subscribers DS-1 point-to-point service, while within the core network, the DS-1 signals are carried as packets.
One of the most promising methods for use in CES is Multiprotocol Label Switching (MPLS). MPLS is described in detail by Rosen et al., in Request for Comments (RFC) 3031 of the Internet Engineering Task Force (IETF), entitled “Multiprotocol Label Switching Architecture” (January, 2001), which is incorporated herein by reference. This RFC is available at www.ietf.org/rfc.html. In conventional IP routing, each router along the path of a packet sent through the network analyzes the packet header and independently chooses the next hop for the packet by running a routing algorithm. In MPLS, however, each packet is assigned to a Forwarding Equivalence Class (FEC) when it enters the network, depending on its destination address. The packet receives a short, fixed-length label identifying the FEC to which it belongs. All packets in a given FEC are passed through the network over the same path by label-switching routers (LSRs). Unlike IP routers, LSRs simply use the packet label as an index to a look-up table, which specifies the next hop on the path for each FEC and the label that the LSR should attach to the packet for the next hop.
Since the flow of packets along a label-switched path (LSP) under MPLS is completely specified by the label applied at the ingress node of the path, a LSP can be treated as a tunnel through the network. Such tunnels are particularly useful in network traffic engineering, as well as communication security. MPLS tunnels are established by “binding” a particular label, assigned at the ingress node to the network, to a particular FEC. Multiple tunnels may belong to the same FEC, but each tunnel will have its own label binding.
When IP packets are passed through a MPLS tunnel, the routing label is removed at the egress node, which then simply routes the packet over its next hop using the packet's IP header. There is no need for the label to tell the egress node what to do with the packet, since the existing IP header, which was applied to the packet before it entered the tunnel, provides all of the necessary information. When layer 2 packets, such as Ethernet frames or ATM cells, are sent through a MPLS tunnel, however, the standard layer 2 media access control (MAC) header that brought the packet to the ingress node does not contain all the information that the egress node requires for delivering the packet to its destination. There is thus a need for a label that tells the egress node how to treat the received packet.
In response to this problem, Martini et al. have proposed to add a “virtual connection” label (or VC label) to the stack of labels used in transporting layer 2 packets through MPLS tunnels. This proposal is described in detail in an IETF draft entitled “Encapsulation Methods for Transport of Layer 2 Frames over MPLS” (May, 2001), which is incorporated herein by reference. The same authors also specify label distribution procedures for binding the VC label to the desired tunnel in a further draft entitled “Transport of Layer 2 Frames over MPLS” (May, 2001), which is likewise incorporated herein by reference. The above documents are available on the IETF Web site.
In accordance with the protocol proposed by Martini et al., before initiating the layer 2 service, the MPLS tunnel is established between the ingress and egress nodes. To set up the required VC label binding for the layer 2 service, the ingress node sends a signaling packet to the egress node carrying a group ID and a VC ID. The group ID represents an administrative group of VCs, and is used for administrative operations on the group. The VC ID is used by the layer 2 service endpoints to associate the locally-configured service with the tunnel.
Malis et al. describe the use of MPLS to carry SONET frames, in an IETF draft entitled, “SONET/SDH Circuit Emulation Service Over MPLS (CEM) Encapsulation” (April, 2001), which is incorporated herein by reference. This document is available at search.ietf.org/internet-drafts/draft-malis-sonet-ces-mpls-05.txt. A SONET OC-N signal is terminated at the packet network ingress node, and the SPE is broken into fragments of fixed length. A CEM header is prepended to each fragment, and a MPLS label stack is then pushed on top of the CEM packet. The MPLS label stack includes the VC label as the last label prior to the CEM header, preceded by an optional tunnel label and additional MPLS labels, depending on how the CEM packets are to be transported through the packet network. At the egress node from the packet network, the CEM packet stream is buffered to absorb delay variations, and the data payload is converted back into a SONET TDM signal using the VC label and CEM header information.
The CEM header defined by Malis et al. contains a number of fields that are used at the egress node to reconstruct the SONET frames. A structure pointer in the CEM header indicates the beginning of the STS-1 POH within the CEM packet payload (or a default value if the fragment does not contain the beginning of the POH). The CEM header also contains N and P bits indicating negative and positive adjustments of the TOH pointer in the STS-1 input frames received at the ingress node. These bits are used to advance or delay the SPE at the egress node in order to preserve the SPE timing between the two circuit emulation endpoints, and thus to reduce the chance of buffer underflow or overflow at the receiving side. In addition, the CEM header includes a sequence number (used to correct packet misordering), an optional error correction code (ECC-6) and a D bit that is used to invoke a dynamic bandwidth allocation (DBA) mode. The DBA mode is used optionally to conserve bandwidth on the packet network by avoiding transmission of trivial SPEs during SONET circuit outages and other abnormal conditions.
Other SONET signals and system features, such as VT sub-rate mapping, are not discussed by Malis et al. in the IETF draft. The authors note that the draft can be extended in the future to support VT and lower-speed non-SONET/SDH services.